Method of using powder metallurgy fabrication for manufacturing integral header and tube replacement sections

ABSTRACT

A method of manufacturing integral head and tube replacement sections includes the steps of providing a reverse mold of a head and tube replacement section, providing an atomized steel powder, and filling the reverse mold with the atomized steel powder. The method further includes the step of inserting the mold into a hot isostatic processing (HIP) furnace to consolidate and sinter the powder into the shape of the head and tube replacement section.

BACKGROUND OF THE INVENTION

This application claims the benefit of Provisional Application No.61/489,505 filed on May 24, 2011.

This application relates to a method of manufacturing integral headerand tube replacement sections.

Many fossil power plants were built for continuous base-load operationand are now beginning to see meaningful cyclic operation. Significantstrains on components such as headers and high temperature piping arecommonly associated with the cyclic practices often resulting incomponent degradation, cracking, and eventual failure of the component.Additionally, cyclic operation can result in thermal gradients atvarious locations along the length of headers which can lead tooverheating and damage at these locations. When damage is encountered,utilities are often faced with the dilemma of replacing the entireheader or removal of a short section (usually on the order of 3-8 feetin length) of the header.

Carbon or low alloy steel and stainless steel headers are commonlyfabricated using either rolled & welded (R&W) plate sections or extrudedpipe sections. Penetrations (or holes) are machined into the header atspecified orientations around the header diameter and along specificlengths wherein stub tubes are then joined to the component. The stubtubes are joined to the header via various welding methods andprocesses, depending on the manufacturer. Due to the geometry of thestub to header weld, the welding is often performed manually andinspection is very difficult. Thus, failures in the weld or ligamentcracking between penetrations are common failure mechanisms.

Acquisitions of replacement sections often require long lead-times, asmuch as 12 months or more, with the manufacturer resulting in de-ratingof the plant and lost revenues until the replacement section or a newheader can be obtained.

BRIEF SUMMARY OF THE INVENTION

These and other shortcomings of the prior art are addressed by thepresent invention, which provides a method of manufacturing integralheader and tube replacement sections that provides a cost-effectivealternative to the conventional manufacturing processes used forproducing a header replacement, that provides for shorter lead-times,and that improves the overall quality of the header replacement sectionas a number of attachment welds (tubes to header) can be eliminated.

According to one aspect of the present invention, a method ofmanufacturing integral head and tube replacement sections includes thesteps of providing a reverse mold of a head and tube replacementsection, providing an atomized steel powder, filling the reverse moldwith the atomized steel powder, and inserting the mold into a hotisostatic processing (HIP) furnace to consolidate and sinter the powderinto the shape of the head and tube replacement section.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter that is regarded as the invention may be bestunderstood by reference to the following description taken inconjunction with the accompanying drawing figures in which:

FIG. 1 shows an integral header to tube attachment according to anembodiment of the invention; and

FIG. 2 is a flow diagram of the method for manufacturing an integralheader to tube attachment.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, an integral header to tube attachment sectionformed in accordance with an embodiment of the invention is illustratedin FIG. 1 and shown generally at reference numeral 10.

The present invention provides an alternative to the conventionalmanufacturing process which results in shorter lead-times and improvedoverall quality of a replacement section. It uses a combination ofpowder metallurgy (PM) and hot isostatic processing (HIP) to generatethe replacement section that is near-net shaped with stub tubes producedas part of the overall PM/HIP effort. No welds are required to join thestub tubes to the header. With this process, the problematic weld jointsare eliminated entirely and result in a much longer overall life of theintegral header assembly. The technology is applicable to headers usedin fossil plants, HRSG plants, and any other applications (chemical,petro-chem, pulp & paper) involving headers and header replacement.

The HIP/PM technology eliminates the rolled & welded or extrusionmanufacturing steps as a header section can be produced as one completedsystem. More importantly, it eliminates the joining of the stub tube tothe header as the stub tube and header are integrally manufactured inone continuous PM/HIP process. Referring to FIG. 2, the process involvesthe design of an exact duplicate, Block 11, of the damaged headersection including the short tube sections, FIG. 1, which can be obtainedfrom drawings of the header. Next, a reverse mold (container) of theheader section is generated, Block 12, in two halves (or more) from acarbon steel material that establishes the final shape of the headersection. The mold is assembled together and then filled with an atomizedlow alloy steel powder to fill the mold, Block 13. Next, the mold isevacuated using a vacuum to eliminate any potential air pockets and thensealed via welding, Block 14.

The entire assembly is then inserted into an HIP furnace and brought toa high temperature and pressure (usually under an inert argonatmosphere) to consolidate and sinter the powder into the final shape ofthe header, Block 16. The assembly is maintained at the sinteringtemperature for a given period of time and then allowed to cool to roomtemperature, Blocks 17 and 18. Additional heat treatment will likely berequired to bring the header to a normalized and tempered condition forservice, Block 19. This final heat treatment can be performed in or outof the mold. Removal of the mold is required once the header has beenallowed to return to room temperature, Block 20.

At this point, the header should be in a near-net shape (near finalshape) condition. Some clean-up and grinding may be required to assurethat the can, mold and any residuals are removed to obtain a finalsurface, Block 21. A couple of additional steps are also required atthis point: 1) boring of the stub tubes to produce an inner penetration,Block 22, and 2) chamfering of the inside diameter of the bore regions,Block 23. Both of these operations are easily accomplished using CNCmilling/boring operations.

It should be pointed out once again that the stub tubes are now anintegral part of the header that requires no weld transition between theheader and stub tube, a region of considerable problems in the past.Elimination of the weldment, removes thermal expansion concerns,potential fatigue and creep damage issues, and wedging that is oftenassociated with the weld attachment of a stub tube. As an integral stubtube, only welds to attach the stub tube to the existing boiler tubesare required, significantly reducing future damage. Because the shapecan be carefully controlled, repeatable smooth transitions between thestub and header are achieved reducing the potential for stress risers.

The foregoing has described a method for manufacturing integral headerand tube replacement sections. While specific embodiments of the presentinvention have been described, it will be apparent to those skilled inthe art that various modifications thereto can be made without departingfrom the spirit and scope of the invention. Accordingly, the foregoingdescription of the preferred embodiment of the invention and the bestmode for practicing the invention are provided for the purpose ofillustration only and not for the purpose of limitation.

1. A method of manufacturing integral head and tube replacementsections, comprising the steps of: (a) providing a reverse mold of ahead and tube replacement section; (b) providing an atomized steelpowder; (c) filling the reverse mold with the atomized steel powder; and(d) inserting the mold into a hot isostatic processing (HIP) furnace toconsolidate and sinter the powder into the shape of the head and tubereplacement section.
 2. The method according to claim 1, furtherincluding the step of subjecting the mold to a vacuum to eliminate airpockets.
 3. The method according to claim 2, further including the stepof sealing the mold to maintain vacuum.
 4. The method according to claim1, further including the step of bringing the HIP furnace up to a highpressure and a high temperature to consolidate and sinter the powder. 5.The method according to claim 4, wherein the HIP furnace is brought upto a high temperature and high pressure in an inert gas atmosphere. 6.The method according to claim 4, further including the step ofmaintaining the HIP furnace at a high temperature and high pressure fora pre-determined amount of time.
 7. The method according to claim 1,further including the step of cooling the mold and sintered powder toroom temperature.
 8. The method according to claim 1, further includingthe step of heat treating the head and tube replacement section.
 9. Themethod according to claim 1, further including the step of finishing thehead and tube replacement section into final form.
 10. The methodaccording to claim 9, wherein the step of finishing includes the step ofgrinding the outer surface of the head and tube replacement section toremove any residuals and obtain a final surface.
 11. The methodaccording to claim 9, wherein the step of finishing includes the step ofboring a stub tube of the head and tube replacement section to producean inner penetration.
 12. The method according to claim 9, wherein thestep of finishing includes the step of chamfering an inside of boreregions of the head and tube replacement section.